Vaughan U.S. Pat. No. 3,155,046, issued Nov. 3, 1964, discloses a centrifugal pump having an open impeller with radial vanes. The vane edges adjacent to the pump inlet cooperate with sharpened edges of inlet apertures to cut stringy material or chunks entering the pump. Similarly, Vaughan U.S. Pat. No. 3,973,866, issued Aug. 10, 1976, and Dorsch U.S. Pat. No. 4,842,479, issued Jun. 27, 1989, disclose centrifugal pumps having impellers with vanes cooperating with inlet apertures to achieve a chopping or slicing action of solid material in a liquid or slurry being pumped. In the case of the pumps of Vaughan U.S. Pat. No. 3,973,866 and Dorsch U.S. Pat. No. 4,842,479, however, closed impellers having radial shroud plates are used; and external booster propellers are provided to accelerate flow into the pump, to displace chunks of solid matter which become lodged in the inlet apertures and, at least in some instances, to cut solid matter prior to entry into the pump.
Other types of pumps having external cutters rotated with an impeller or propeller are shown in Farrand U.S. Pat. No. 2,714,354, issued Aug. 2, 1955; Peterson U.S. Pat. No. 3,325,107, issued Jun. 13, 1967; and French Patent No. 1.323.707, issued Mar. 1, 1962.
Sutton U.S. Pat. No. 3,444,818, issued May 20, 1969, discloses another type of centrifugal pump having an internal impeller with vanes cooperating with the periphery of an inlet aperture to achieve a slicing action. In the Sutton construction, an outer "chopper member" has blades that wipe across the outer surface of the apertured intake plate to assist in chopping solid material to a size small enough to enter the intake aperture. Similarly, in the construction shown in British Patent No. 1,551,918, published Sep. 5, 1979, external blades sweep across small intake apertures to dislodge or gradually cut solid material clogging an intake aperture. In both the construction shown in the Sutton patent and the construction shown in the British patent, the external member is mounted so as to be moveable axially away from the intake plate if a hard obstruction is encountered.
Other types of pumps designed for pumping liquids or slurries containing solid materials are disclosed in Canadian Patent No. 729,917, issued Mar. 15, 1966; Schlesiger U.S. Pat. No. 3,340,812, issued Sep. 12, 1967; Elliot U.S. Pat. No. 4,527,947, issued Jul. 9, 1985; and Corkill U.S. Pat. No. 4,575,308, issued Mar. 11, 1986.